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Date: Tue, 5 Aug 2008 04:18:12 +0200 From: Andrea Arcangeli <andrea@...ranet.com> To: Roland Dreier <rdreier@...co.com> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>, Dave Jones <davej@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, David Miller <davem@...emloft.net>, jeremy@...p.org, hugh@...itas.com, mingo@...e.hu, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, arjan <arjan@...radead.org> Subject: Re: [PATCH] workaround minor lockdep bug triggered by mm_take_all_locks On Mon, Aug 04, 2008 at 07:00:03PM -0700, Roland Dreier wrote: > > The point is that this is a runtime evaluation of lock orders, if > > runtime isn't the lucky one that reproduces the deadlock, it'll find > > nothing at all. > > I think the point you miss is that lockdep can report a potential > deadlock, even if the deadlock does not actually occur. For example > suppose there is an AB-BA deadlock somewhere. For this to actually > trigger, we have to have one CPU running the AB code path at exactly the > moment another CPU runs the BA code path, with the right timing so one > CPU holds A and tries to grab B while the other CPU already holds B. > > With lockdep, we just have to have the AB code path run once at any > point, and then the BA code path run at any later time (even days after > the AB code path has released all the locks). And then we get a > warning dump that explains the exact potential deadlock. Thanks a lot for the detailed explanation of check_noncircular. I agree check_noncircular is surely a good argument not to get rid of prove-locking as a whole. But check_noncircular is also a red-herring in this context. It's not check_noncircular trapping here, check_deadlock traps with false positives instead. The question is what are those false positives buying us? To avoid a developer to press sysrq+p or break on kgdb? Let's focus on check_deadlock->print_deadlock_bug and somebody who's not beyond the point please explain what print_deadlock_bug reports that does not actually occur and why it's a good idea to change the common code to accommodate for its false positives instead of getting rid of it for good. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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